It is necessary to produce periodically an essentially disinfected condition in soft contact lenses so that bacterial organisms or their by-products will not cause harm to the wearer's eyes. Since the soft lens material is permeable to liquids, soaking the lens in strong germicidal solutions will result in the lens becoming impregnated with the solution, and this can lead to irritation to the user's eye when the lens is worn. In general, it has been found difficult to disinfect soft lens by treatment with chemical or biochemical solutions which will not cause eye irritation to at least some percentage of the wearers.
As an alternate means for producing the desired disinfected condition in the soft lens, heat may be used. The lens may be kept immersed in physiologically normal saline solution, or its equivalent, when it is not being worn, to prevent the lens material from drying out. Heat is therefore generally applied by first placing the lens in a suitable container or lens holder; adding a suitable amount of saline solution, of the proper concentration, so that the lens is totally immersed; closing or covering the lens container; placing the lens container in a suitable heating unit; and energizing the heat unit.
The heating unit must raise the temperature of the saline solution and immersed lens to the required temperature, hold the lens at or above this temperature for the required time, and then allow it to cool to ambient temperature. Typical values of the time and temperature deemed suitable for producing the disinfected condition require the lens to be maintained at or above 80.degree. C., for a period of 10 minutes or more. Since aging of the lens material is accelerated by excessive temperatures and/or by extended time at elevated temperatures, it is desirable that the heating unit be controlled so that excessive temperatures, or excessive time at elevated temperatures, will not shorten the life of the lens.
The heating unit must be designed with careful attention to electrical safety/shock hazard considerations since the user typically uses the unit in a bathroom adjacent to grounded water piping and wash basins. Since the typical household electrical outlets are only two contact, without grounding pin provisions, it is impractical to use a 3-wire power cord with a ground connection to any exposed metallic portions of the device. The user of the prior art unit is therefore potentially subject to a significant shock hazard if he touches a grounded object such as a water faucet while in contact with any metallic portion of the heating unit. Any current leakage path between the electrical circuitry and the exposed metallic portion of the prior art unit can lead to potential fatal shock hazards under these conditions. It would therefore be very advantageous to have the heating unit constructed so that there is no exposed or exposable portion of the unit constructed of metal or other electrically conductive materials. The unit of the present invention is so constructed.
There are two general design approaches which have been used in the prior art to produce heating units for this type of application. The first is to incorporate a metallic heating block which has been configured to hold the lens container. An electrical heater in the form of a surface mounting flat heater mounted to a flat face of the block, or in the form of a tubular heater fitting into a hole or slot, is used to provide heat input to the block. A thermostatic switch, connected in series with the heater regulates the block temperature at some selected set point. The timing function is typically provided by a separate spring wound clock-work or motor driven timer. An overall plastic heating is usually used to provide thermal isolation for the metal heating block and to enclose the electrical components.
The disadvantages of the prior art construction described in the preceding paragraph are numerous. The metallic heating block requires relatively expensive precision casting and/or machining operations, as well as requiring added cost in painting or other finishing operations. The heating block provides a potential shock hazard should any electrical leakage develop between it and the electrical circuitry. It is difficult to provide a liquid tight seal between the metallic heating block and the plastic enclosure so that any liquid spilled or splashed in the use of the unit is likely to cause damage to, or current leakage in the unit. There is typically no way for the user to tell if the unit has reached its proper operating temperature. The cost of the timer incorporated into the prior art unit is appreciable and adds significantly to the relatively large manufacturing cost of this type of unit.
The heating element, either flat or tubular, is again of significant cost in the aforesaid prior art unit. Since the total timer cycle includes the time necessary to raise the heating block temperature to the regulating point of the thermostat, differences in input power due to heater resistance tolerances, or to variations in the applied power line voltage, will cause variations of the warm-up time and thus to the remaining time-at-tempeature cycle of the timer.
The second prior art design approach is again one where a metallic heating block is used. The configuration of the heating block is similar to that of the first approach and the same type of electrical heaters may be used. A thermally sensitive switch is mounted on the block and connected in series with the heater. The switch is manually actuated to energize the unit. The heating block serves as a conductive means to distribute the heat from the heating element to the lens and to the thermal switch. When the switch temperature reaches its preset actuating temperature, the switch opens and removes power from the heating element. The block temperature immediately starts decreasing towards the ambient temperature. As the block is being heated, a certain amount of heat energy is stored in the thermal capacity of the block. The mass of the block, its specific heat, and its temperature rise determine the amount of heat energy stored. The amount of stored heat, and the insulating factors of the block mounting in the usual plastic case, determine the rate of decay of the block temperature.
In order to maintain the lens temperature at or above a given value for a given time in the second type of prior art heating unit, it is usually necessary to allow the lens temperature to overshoot the desired value by an appreciable amount in order that sufficent time at or above the desired temperature can be realized.
All of the disadvantages of the metallic heating block mentioned in the first prior art unit apply equally to the second prior art unit. The heating element is again a siginificant cost factor. There is typically no way for the user to be certain that the second prior art unit has attained the proper temperature in use. Only a limited amount of heat energy can be stored in the thermal mass of the heating block of the second prior art unit so it is hard to maintain the lens at a more-or-less constant temperature.